There are a number of communication links, such as HF transmission paths, that introduce distortion or contamination into messages transmitted over the link and thereby make it difficult to faithfully reproduce the original message at the receiver. Transmission distortion may be the result of any number of effects, such as multi-path reception, group delay distortion, noise amplitude distortion, interference, dispersive fading or, in general, a time-spread of the time response of the overall communication link. Because of these adverse influences, the data received from the transmission medium is seldom the same as the original message, so that some form of anti-distortion compensation scheme must be employed in the receiver's signal processing equipment.
One approach is the use of adaptive equalizers or recursive filters through which the characteristics of the channel are estimated and the data is subjected to a predistortion filter function based upon the observed influence of the channel on the signals as they are received. For an overview of various proposals dealing with recursive filter/equalization implementations attention may be directed to the description of the various techniques described in U.S. Pat. No 4,038,536 to Feintuch. While these systems seek to adapt themselves to the distortion introduced into the data, they rely on unknown signal inputs and suffer from a degree of estimation or inaccuracy that limits the data rate to which they may be applied.
Another scheme, described in U.S. Pat. No. 4,058,713 to Di Toro, seeks to provide adaptive equalization to compensate time spread and frequency spread distortion, and for this purpose, employs a known signal that is interleaved with segments or portions of the original message at the transmitter, thereby creating alternating bursts of unknown data and known signal spaced apart in time and transmitted to the receiver. At the receiver, rather than employ a time domain equalization approach, the distortion compensation equipment converts the received message signal and a known test signal into the frequency domain for processing, in order to obtain an estimate of the transfer function of the communication link which can then be employed to recreate the originally transmitted message signal. Unfortunately, with this type of approach, there is a hiatus or delay factor involved in the format of the transmitted message, which not only limits the data rate, but may cause errors in the signal recovery process due to changes in the transmission channel.